The present invention relates to a battery package including opposite polarity electrical connectors for connecting at least one electrochemical cell to an external load, at least one of the opposite polarity electrical connectors being adapted to automatically shut down the battery in response to a sufficient increase in internal pressure within the battery package.
Non-aqueous electrochemical cells have become the subject of increasing study and development in recent years, owing to their advantages over conventional electrolyte batteries. Particularly promising are non-aqueous cells comprising a cathode including a metal oxide, chalcogenide, or phosphate active material, for instance a lithium metal oxide, a polymer electrolyte, and an anode including an active material capable of storing or releasing ions during cell operation. The particular advantages of non-aqueous metal oxide electrochemical cells include lower weight than conventional liquid-electrolyte batteries, long service life, relatively high energy densities, and relatively high specific energies.
A typical non-aqueous electrochemical cell, such as a rechargeable lithium ion cell, includes, sequentially, a cathode, a separator, and an anode sandwiched together to form the cell. This cell precursor can be extracted and activated with electrolyte to form a functional cell. More particularly, an electrolyte salt solution is imbibed into a polymeric matrix separator, yielding the xe2x80x9cactivatedxe2x80x9d functional cell.
A typical electrochemical battery comprises several such electrochemical cells in which the current from the opposite polarity electrodes of each cell is accumulated by current collectors.
Presently favored electrochemical cell types include the xe2x80x9cbi-cell,xe2x80x9d characterized by a central electrode (either anodic or cathodic) flanked by two counter-electrodes. Specifically, a conventional bi-cell comprises, in sequence, a first counter-electrode with a current collector, a first separator, a central electrode with a current collector, a second separator, and a second counter electrode having a current collector.
In order to connect an electrochemical cell to an external load, the cell is provided with electrically conductive connectors or tabs associated with the opposite polarity electrodes of the cell. Typically, the current collectors of an electrochemical cell include integral tab portions or separate, primary current collectors associated with the cell current collectors and comprising tab portions for connecting the cell to an external load. Exemplary electrical connectors for an electrochemical cell stack are described in U.S. Pat. No. 5,300,373, assigned to Valence Technology, Inc., which disclosure is incorporated herein by reference in its entirety.
Electrochemical cells, including bi-cells, may be packaged in flexible, bags, pouches, or other containers. According to this type of packaging, the electrochemical cell is essentially sealingly enclosed within the packaging and a portion of the electrical connectors of the cell protrude therefrom to permit electrical contact between the connector and an external load. The packaging forms an essentially sealed enclosure which impedes or prevents infiltration of air and/or moisture into the package. Exemplary layered laminate packaging materials include, but are not limited to, multilayer plastics and barrier materials described in U.S. Pat. Nos. 4,997,732 and 5,445,856, incorporated herein by reference in their entirety. Such materials prevent, or at least inhibit, transport of electricity, oxygen and water therethrough.
One common problem with the aforementioned flexible type packaging is that as an electrochemical cell is charged above its normal voltage limits (e.g, 4.2V for spinel cathode/graphite anode cell), a number of destructive reactions take place within the cell, releasing unwanted gaseous by-products. With the production of these gases, both pressure and temperature increase within the cell package. Continued gas generation ultimately results in package distortion, cell damage, and even short-circuiting. In the extreme case of a short-circuit, the resultant spark or increased temperature can ignite the gas, thereby causing a fire.
Production controls in the electrochemical cell industry, such as imposed by Underwriter""s Laboratories (xe2x80x9cULxe2x80x9d), require overcharge testing of flexible-package batteries. For instance, UL overcharge test 1642 requires the cell to be charged to 12 V at a current three times the manufacturer""s maximum recommended charging rate. It has proven practically very difficult for flexible-packaging cells to pass these overcharge tests for the aforementioned reasons.
Consequently, it would be expedient to have an effective means of preventing cell damage and short circuiting resulting from overcharging, and to otherwise improve the safety of flexible package type batteries.
It is accordingly one object of the present invention to provide an improved flexible package type battery whereby the battery is able to automatically shut-down (i.e., become incapable of powering an external load) before the consequences of unwanted pressure build-up (for instance, as a consequence of overcharging) result in battery damage or other undesirable effects.
It is a further object of the present invention to be able to control such automatic shut-down so that the battery is rendered inoperative at a point prior to irreversible battery damage caused by unwanted pressure build-up.
The foregoing objects are achieved through a flexible-package type battery characterized in that the electrochemical cell (which may comprise, for instance, a bi-cell) is automatically rendered inoperative in the event of a sufficient increase in internal pressure within the battery package. This object is achievable by the present invention because, unlike prior art flexible-package type batteries, the battery disclosed herein comprises at least one electrical connecter adapted to automatically shut down the battery in response to a sufficient increase in internal pressure within the package.
According to one feature of this invention, the at least one electrical connector does not comprise unitary, electrical connector fixedly connected to the electrochemical cell and extending therefrom through the package so as to be accessible from the exterior of the package, as is the case with prior art flexible-package type batteries. Rather, the battery of this invention is characterized in that at least one of the electrical connectors comprises separate electrically conductive elements. According to this feature, the battery is further characterized by a first, operative condition wherein these separate electrically conductive elements are in electrically conductive contact, and a second, inoperative condition wherein the separate electrically conductive elements are out of electrically conductive contact in response to a sufficient increase in pressure within the package.
According to another feature of this invention, the first, operative condition of the battery is further characterized by the creation of a sufficient vacuum in the interior volume of the battery package to bring the separate electrically conductive elements into electrically conductive contact.
The electrically conductive elements of the inventive battery may comprise a cell tab associated with the electrochemical cell and disposed entirely within the battery package, and a package tab having a first portion extending into and associated with the package, and a second portion extending outside of the package to connect the battery to an external load. According to this feature, the creation of a sufficient vacuum in the interior volume of the package decreases the internal volume, thus bringing the package tab into electrically conductive contact with the cell tab to render the battery operative. Upon an increase in pressure, for instance as a result of overcharging, the internal volume of the package increases with increasing pressure until the pressure increases is sufficient to move the package tab and cell tab out of electrically conductive contact, thereby rendering the battery inoperative.
According to a further feature of this invention, the separate electrically conductive elements are adapted to remain in electrically conductive contact until the internal pressure in the package increases beyond a predetermined amount. This may be accomplished, for instance, by providing either (or both) of the cell and package tabs with a sprung portion; that is, a portion of the electrically conductive tab adapted to engage the other tab in electrically conductive contact even as the tabs are moved apart in response to an increase in internal pressure in the package.